Method of high resolution of electrostatic transfer of a high density image to a receiving substrate

ABSTRACT

A method of fabricating a toned pattern on an isolated nonabsorbent receiving surface is disclosed wherein a charged electrostatic latent image area is established on an electrostatically imageable surface, and is transferred to the receiving surface across a gap of between about 1 mil and about 20 mils filled with a liquid formed at least partially of a nonpolar insulating solvent in which are suspended charged toner particles.

This application is a continuation-in-part of application Ser. No.158,168 filed Nov. 19, 1987, now U.S. Pat. No. 4,786,576 issued Nov. 22,1988; which is a continuation of application Ser. No. 883,797 filed July9, 1986, now abandoned; which is a continuation of application Ser. No.848,669 filed Apr. 4, 1986, now U.S. Pat. No. 4,661,431 issued Apr. 28,1987; which is a continuation-in-part of application Ser. No. 655,346,filed Sept. 27, 1984, now abandoned.

BACKGROUND OF THE INVENTION

This invention relates generally to a method of high resolutionelectrostatic transfer of a high density image to a nonabsorbentreceiving surface. More specifically, it pertains to the method oftransfer and the method of creating a latent image on anelectrostatically imageable surface that may be repeatedly used toproduce high resolution and high density nonconductive images onreceiving surfaces. Density as used herein with respect to nonconductiveimages refers to the number of individual images per unit surface area.

The production of conductive wiring patterns on an insulating substrateemploying a dry film resist by use of photoimaging and other techniquesto produce a printed circuit board typically employs a five stepprocess. Regardless of whether a tenting method or a hole-pluggingmethod is employed, the five distinct steps have included laminating orcoating a photosensitive dry film resist on at least one conductivesurface of an insulating substrate, forming a wiring pattern on the dryfilm resist by use of artwork or a phototool and exposing the dry filmresist to actinic radiation through the transparent areas of thephototool, developing the circuit board by removing the unexposedportions of the negative working dry film resist, etching the conductivesubstrate from the circuit board in all non-imaged areas not beneath thedesired conductive wiring pattern which is still covered with the dryfilm resist, and finally stripping or removing the dry film resistcovering the desired wiring pattern from the non-etched portions of theconductive substrate. This five step process must be repeated for eachcircuit board produced.

During the exposure step in the standard dry film process, sufficientradiation exposure levels and exposure times are desired to producestraight sidewalls in the dry film resist that are the result of apattern of the cross-linking of polymers in the dry film. These straightsidewalls should be normal to the conductor surface. Practically,however, in the standard negative working dry film photoresist print andetch process either underexposure occurs, producing a sidewall edge thatundercuts the desired resist pattern, or overexposure occurs, producinga sidewall edge in the dry film photoresist that increases the width ofthe dry film photoresist at the base of the resist and the surface ofthe conductor causing a foot. Both of these conditions vary the width ofthe ultimate conductive pattern from that which is desired, beyond theplanned and engineered tolerance or overage of the line widths in theconductor surface.

The development step during this process ideally should develop away theunexposed negative working dry film resist to produce an edge in the dryfilm resist on the conductor surface that is equal in width to thepattern on the phototool and normal to the conductor surface.Practically, however, either underdevelopment or overdevelopment of thedry film photoresist occurs. Underdevelopment produces a buildup ofresist residue in the sidewall zone or developed channels that is slopedtoward the adjacent sidewall resulting in smaller spaces between theadjacent lines than is desired. When overdevelopment occurs theunexposed film resist edge is undercut, producing larger than desiredspacing between adjacent lines. Additionally, there is the potential forsome rounding at the top of the resist surface sidewall edges.

This inability to accurately reproduce the phototool in the dry filmresist affects the fine line resolution and reproduction characteristicsof the reproduced circuit pattern. As circuit boards have become morecomplex and stacking of multiple boards has become prevalent, the needfor higher density, finer resolution circuit patterns has evolved.Resolution has been viewed as the ability to reliably produce thesmallest line and space between adjacent lines that can be reliablycarried through the aforementioned five step processing. The thinness orsmallness of the lines that can survive development and the narrownessof the gap or space between the adjacent lines in the circuit patternhave led to fine line resolution and reproduction standards in theprinted circuit board industry calling for about 3.1 mil line and spacedimensions or the development of about 6.3 line pairs per millimeter.These standards are used to define the desired density of the circuitboard.

The attempt to apply the principles of xerography to transfer developedelectrostatic latent images from a photoconductor's electrostaticallyimaged surface to a receiving surface with high resolution and highdensity images has encountered difficulty. The major source of thisdifficulty stems from the fact that circuit boards consist of anonporous or nonabsorbent substrate, such as metal, like copper, or aplastic, like the polyester film sold under the tradename of MYLAR. Thisnonporous and nonabsorbent receiving surface causes the image beingtransferred, especially when attempted with a liquid toner, to becomedistorted or "squished".

Xerographic techniques solved the problem of transferring an image toabsorbent receiving surfaces, such as paper, by transferring the imagesformed by toner particles across a gap. The gap has either been an airor a combination air-liquid gap. Attempts to translate this gap transfertechnology to nonporous substrates, however, resulted in image "squish"and the realization that the gap space and the voltage must be carefullycontrolled to produce an acceptable transferred toner image with theproper resolution and density. If the voltage and the gap space ordistance between the photoconductor or the electrostatically imageablesurface and the conductive receiving surface are not carefullycontrolled, electrical arcing across the gap will occur. This can causepin-holes in the transferred toner image by permanently damaging theelectrostatically imageable surface. This is especially significant inprint and etch applications used to manufacture printed circuit boards.

Also, it has been found with nonabsorbent receiving substrates that boththe photoconductor or electrostatically imageable surface and thereceiving surface must be stationary at the point of transfer of thetoner image to achieve a transferred image of high resolution.

An additional problem is presented in transferring the developed latentimage electrostatically to a nonabsorbent substrate, such as copper. Themetal or copper surface forming the conductive receiving surface, aswell as the electrostatically imageable surface, is uneven so that thespacing between the electrostatically imageable surface and theconductive receiving surface must be sufficient to avoid contact betweenthe uneven surfaces of the photoconductor and the conductive receivingsurface.

Regardless of whether the receiving surface is conductive ornonconductive, the key in effecting a transfer is establishing asufficient electric field between the electrostatically imageablesurface and the receiving surface.

These problems are solved in the process of the present invention byproviding a method of making a transfer of a developed electrostaticlatent image from an electrostatically imageable surface across aliquid-filled gap to a nonabsorbent receiving surface to producemultiple pattern copies from a single permanent latent image. Theelectrostatically imageable surface may be either a photoconductor or apermanent master.

SUMMARY OF THE INVENTION

It is an object of the present invention to provide a method forachieving non-contact high resolution electrostatic transfer of adeveloped high density electrostatic latent image directly to anonabsorbent substrate.

It is another object of the present invention to obtain the high densityelectrostatic latent image through the use of a photopolymer, such as adry film resist, that serves as a permanent and reuseable master.

It is another object of the present invention that the method ofelectrostatic transfer also can be utilized with a photoconductor or apermanent master as the electrostatically imageable surface.

It is another object of the present invention to transfer the developedhigh density image from the electrostatically imageable surface across aliquid-filled gap to a receiving surface so that the liquid serves asthe transfer medium.

It is still another object of the present invention to provide a methodthat permits the latent image and the transferred image to be capable ofresolving about 3.1 mil line and space.

It is a feature of the present invention that the quality of the imagedensity, the thickness or height of the toner particles forming theimage and the thickness of the layer of liquid serving as the transfermedium in the gap between the electrostatically imaged surface and thereceiving surface are controlled by the spacing of the gap and thevoltage applied to create the electric field between theelectrostatically imageable surface and the receiving surface.

It is another feature of the present invention that the conductivebacking material or the substrate supporting the electrostaticallyimageable surface is electrically grounded and the receiving surface iselectrically isolated from ground.

It is still another feature of the present invention that the developedlatent image is transferred directly from the electrostaticallyimageable surface to the nonabsorbent receiving surface through anonconductive dielectric insulating fluid by the migration of theindividual toner particles comprising the toned image through the liquidto the receiving surface giving a more uniform pile height between thecenter and the edge of the image.

It is yet another feature of the present invention that the distance orspacing of the gap between the electrostatically imageable surface andthe receiving surface is between about 1 mil and about 20 mils and thedistance is maintained by the use between the two surfaces of spacermeans which are electrically isolated from ground.

It is still another feature of the present invention with the use ofconductive receiving surfaces that conventional photoconductors orpermanent masters may be used as the electrostatically imageable surfaceto produce large quantities of printed circuit boards by eliminating theneed for a dry film or liquid photoresist for each circuit board copy.

It is yet another feature of the present invention that the transfer ofthe electrostatic latent image to the receiving surface is accomplishedby directly applying D.C. voltage to the receiving surface, rather thancorona charging, or where the receiving surface is a dielectric materialpositioned atop of a conductive supporting substrate, directly to theconductive supporting substrate.

It is a further feature of the present invention with a photoconductorused as the electrostatically imageable surface that an additionalexposure is required to produce each additional latent image.

It is an advantage of the method of the present invention that highresolution transfer of the toner particles forming the developed latentimage is obtained on the receiving surface without image distortion dueto the electrical lines of force or pathways of toner transport beingsubstantially straight and parallel, thereby giving the more uniformpile height between the center and the edge of image.

It is another advantage of the present invention that there is no damageor abrasion to the electrostatically imageable surface during theprocess so that the surface may be continually reused.

It is still another advantage of the present invention that highresolution transfer is achieved because there is no contact between thedeveloped toner particles on the electrostatically imageable surface andthe receiving surface.

It is yet another advantage of the present invention that the powerrequirements can be reduced to accomplish the electrostatic transferbecause of the use of direct applied voltage, rather than coronacharging which causes air ionization.

It is still another advantage of the present invention that a faster andlower cost method of making printed circuit boards is achieved becauseof the elimination of the repeated exposure and development stepsrequired of dry film or liquid photoresists for each circuit board.

These and other objects, features and advantages are obtained by the useof the method of fabricating a toned pattern on an isolated nonabsorbentsurface by first establishing a developed electrostatic latent image onan electrostatically imageable surface, developing the latent image withcharged toner particles, and then transferring charged toner particlesacross a liquid-filled gap comprised at least partially of a nonpolarinsulating solvent to create an imaged area on a receiving surface whilethe gap is maintained between at least about 1 mil and about 20 mils atthe point of transfer of the toner particles forming the image.

BRIEF DESCRIPTION OF THE DRAWINGS

The objects, features and advantages of the invention will becomeapparent upon consideration of the following detailed disclosure of theinvention, especially when it is taken in conjunction with theaccompanying drawings wherein:

FIG. 1 is a diagrammatic illustration of the prior art print and etchprinted circuit board fabrication steps;

FIG. 2a is a diagrammatic illustration of the process of the presentinvention employing a permanent master that is reusable to producemultiple copies of a desired pattern on an insulating dielectric layerby the migration of charged toner particles from the master across aliquid-filled gap to a conductive receiving surface;

FIG. 2b is an enlarged diagrammatic illustration of the migration ofcharged toner particles across a liquid-filled gap to the conductivereceiving surface of FIG. 2a;

FIG. 3a is a diagrammatic illustration of the process of the presentinvention employing a permanent master that is reusable to producemultiple copies of a desired pattern on an insulating dielectric layerby the migration of charged toner particles from the master across aliquid-filled gap to a non-conductive receiving surface having anunderlying conductive supportive substrate; and

FIG. 3b is an enlarged diagrammatic illustration of the migration ofcharged toner particles across a liquid-filled gap to the non-conductivereceiving surface of FIG. 3a.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

FIG. 1 shows the standard five step process that has been previouslyemployed in the production of printed circuit boards. Each one of thecircuit boards produced has routinely required the application of a dryfilm to a conductive substrate, such as copper, that is laminated to anonconductive substrate, such as fiberglass epoxy, with pressure andheat. A mask is then applied over the dry film to permit selectiveexposure from a light source or other source of actinic radiation toproduce the desired pattern. Development takes place by removing theuncross-linked dry film, leaving only cross-linked dry film with thedesired pattern. Etching with an acid etchant removes the conductivecopper substrate from between the areas of cross-linked dry film.Finally, stripping the dry film from the remaining conductive coppersubstrate exposes the desired circuit pattern. This is commonly known asthe print and etch process.

In the process of the present invention, however, a permanent master isproduced with the use of a photosensitive material or coating that is aphotopolymer, such as a dry film or liquid photoresist, over aconductive substrate. Thereafter, dry film or liquid photoresist is notemployed to produce the desired patterns from the permanent master onthe product receiving surfaces.

The permanent master is used as an electrostatically imageable surfaceas shown in FIG. 2. A conductive backing has a photosensitive material,such as a photopolymer, for example a dry film or liquid photoresist,applied to it on at least one side. This photosensitive materialundergoes a change in resistivity upon exposure to actinic radiationbecause of the cross-linking of the polymers in the material. Apersistent image, essentially a permanent image, is formed on thephotosensitive material by actinicly radiating through a mask or by"writing" the desired pattern with a digital laser pen. Either methodproduces electrostatic contrasts or differences in the resistivitybetween imaged and non-imaged areas on the photosensitive material. Theelectrostatically imageable surface is isolated from ground and chargedwith a corona charging device to produce the charged latent image.

The electrostatically imageable surface is then developed by theapplication, through surface adsorption, of a liquid comprised at leastpartially of a nonpolar insulating solvent that serves as a liquidcarrier for toner particles that are charged oppositely to the charge ofthe electrostatically imageable surface. This application can beaccomplished by flooding, dipping or spraying the electrostaticallyimageable surface. The charged toner particles are directed to thelatent image area of the electrostatically imageable surface to form ordevelop the latent image.

Thus developed, the image is formed on the electrostatically imageablesurface according to the persistent latent image's pattern on thepermanent master. The developed image thus is ready for transfer to anelectrically isolated receiving surface to produce the desired pattern.The suitable receiving surface possesses the characteristics of beingresistant to the nonpolar insulating solvent used to coat it,temperature stable, dimensionally stable and have good toner releasecharacteristics. Suitable receiving surfaces include dielectrics such assilicone, polyethylene terephthalate, or polyvinyl fluoride positionedover a supporting conductive substrate such as copper or aluminum,conductive metals, or conductive silicone elastomer.Polytetrafluoroethylene could be used in place of silicone, althoughfluorosilicone could also be used. Conductive silicone or conductivefluoro-silicone, meaning silicone or fluorosilicone filled with aconductive or semiconductive material, could be laminated over a metalelectrode or to a flexible heat resistant polymer film, such as thatsold under the tradename KAPTON by E. I. Du Pont de Nemours, with anunderlying conductive base. With these receiving surfaces, the developedimage would be transferred through a liquid-filled gap, as discussedpreviously and hereafter, dried and then transferred in a second step byheating the receiving surface.

This transfer of a liquid electrophotographic toner across such aliquid-filled gap is dependent upon the establishment of a sufficientelectric field between the electrostatically imageable surface and thereceiving surface. It is not apparently dependent upon the receivingsurface being classified as conductive.

The receiving surface is first coated with a liquid that comprises atleast partially a nonpolar insulating solvent. The solvent is the sameas or an equivalent to that which is applied to the electrostaticallyimageable surface and may be applied by sponge, squeege, rubber rolleror other means capable of applying a thin continuous film. The solventshould preferably have a high resistivity and a low viscosity to permitthe charged toner particles to migrate or flow through the solvent fromthe charged electrostatic latent image area on the electrostaticallyimageable surface to the receiving surface. The solvents are generallymixtures of C₉ -C₁₁ or C₉ -C₁₂ branched aliphatic hydrocarbons soldunder the tradename Isopar G and Isopar H, respectively, manufactured bythe Exxon Corporation, or equivalents thereof. The electricalresistivity is preferably on the order of at least about 10⁹ohm-centimeters and the dielectric constant preferably is less thanabout 31/2. The use of nonpolar insulating solvents with thesecharacteristics helps to ensure that the pattern of charged tonerparticles is not dissipated.

After a D.C. voltage, optimally between about 200 to about 1200 volts,is applied to the receiving surface or the underlying conductivesupporting substrate to establish an electric field between theelectrostatically imageable surface and the receiving surface, thesurfaces are moved close enough together to create a completely liquidtransfer medium by the contact of the two layers of nonpolar insulatingsolvent. The first liquid surface of the first layer of nonpolarinsulating solvent on the electrostatically imageable surface and thesecond liquid surface of the second layer of nonpolar insulating solventon the receiving surface join together to fill the gap between the twosurfaces. The voltage necessary to establish the electric field betweenthe electrostatically imageable surface and the receiving surfaceoperably can be between about 200 to about 3500 volts, but is preferablybetween about 200 and about 1500 volts and optimally is as stated above.The ability to transfer a high resolution image is a function of thecombined factors of the toner, the liquid carrier, the gap spacing andthe voltage applied. Generally, a greater gap spacing requires a highervoltage to effect a high quality, high resolution image transfer.

A uniform spacing across this gap is maintained by the use of spacerstrips or gap spacers, seen in FIG. 2, which are electrically isolatedfrom ground. The developed image from the electrostatically imageablesurface is transferred across the gap through the liquid medium to thesurface to form an imaged area in a pattern similar to that of thephototool where the transferred toner particles are present andnon-imaged areas where the particles are absent.

The transfer of the developed image across the liquid-filled gap takesplace at the point of transfer by maintaining a first plane takenthrough the electrostatically imageable surface parallel to a secondplane taken through the receiving surface. The electrostaticallyimageable surface and the receiving surface at the point of transfershould have no relative motion occuring between them, although the pointof transfer could be a stationary or rolling point of transfer. A drumor web, or a stationary flat surface could be employed for theelectrostatically imageable surface, transferring the developed imageacross the gap to a flat and stationary, or a moving receiving surface.The moving receiving surface could be a rolling drum or a web or otherappropriate means. The electrostatically imageable and receivingsurfaces must be held in place at the point of transfer, such as by avacuum, or alternately could be accomplished by magnetically orelectrostatically holding the surfaces in place across the gap.

This gap between the electrostatically imageable surface and thereceiving surface is preferably maintained between at least about 3 mils(0.003 inch) and about 10 mils (0.010 inch) by the use of spacer stripsof the desired thickness, although high quality images have beentransferred across gaps as large as about 20 mils (0.02 inch). Bymaintaining the gap greater than about 3 mils, the inconsistencies orirregularities in the two surfaces are separated sufficiently to preventany contact from occurring between the two surfaces and any possibleabrasion or scratching from occurring to the surface of the master orelectrostatically imageable surface.

The spacer strips or gap spacers are selected from either conductivematerials, such as metal, or nonconductive materials, such as polyesterfilm sold under the tradename MYLAR, or cellophane. The strips must beelectrically isolated from ground and be of uniform thickness. Theuniform thickness insures that a uniform gap spacing is obtained betweenthe electrostatically imageable surface and the receiving surface. Thespacer strips preferably should be placed outside of the image area.

By applying the first and second layers of nonpolar insulating solventin sufficient thickness to the electrostatically imageable surface andthe receiving surface to fill the gap therebetween, the first liquidsurface and the second liquid surface of the first and second layers ofthe nonpolar insulating solvent join together to form a continuousliquid transfer medium at the point of transfer of the charged tonerparticles between the electrostatically imageable surface and thereceiving surface. By traveling through a continuous sea of liquidtransfer medium, there are no surface tension forces which the chargedtoner particles must overcome that could hinder their migration from theelectrostatically imageable surface to the receiving surface. Thecharged toner particles are directed through the liquid transfer mediumformed by the joining of the two layers of nonpolar insulating solventat this point of transfer by the electric field that is applied at thepoint of transfer.

As is diagrammatically illustrated in FIGS. 2a and b, and 3a and b, thecharged toner particles with their predetermined charge, migrate fromthe oppositely charged cross-linked imaged area with the photosensitivematerial on the electrostatically imageable surface to the receivingsurface as individual or grouped particles. In the case of a conductivereceiving surface, as seen in FIGS. 2a and b, the receiving surface islaminated onto an insulating dielectric layer, such as a fiberglassepoxy. In the case of nonconductive receiving surfaces, the receivingsurface is positioned over a supporting conductive substrate, as seen inFIGS. 3a and b. The applied electric transfer field causes the tonerparticles to migrate through the liquid transfer medium of the nonpolarinsulating solvent and attach to the receiving surface to create imagedareas where the toner particles are present and non-imaged areas wherethey are absent.

Since the photosensitive material, such as a dry film or liquidphotoresist, on the electrostatically imageable surface acts as a masterelectrostatic image plate, and the resistivity difference between theimaged and non-imaged areas on the electrostatically imageable surfaceremains relatively constant in most instances for sustained periods oftime dependent upon the photoresist used, multiple copies can be made bythe electrostatic transfer method. To repeat the procedure, excessnonpolar insulating solvent and excess toner particles on theelectrostatically imageable surface should be removed, such as byrinsing, followed by a physical wiping or squeeging. Any residualelectric charge on the electrostatically imageable area should bedischarged, such as by charging the photosensitive material's surfacewith an alternating current corona.

The desired electrostatic latent image pattern remains in thephotosensitive material by using the material's ability to retaindifferences in resistivity for relatively long periods of time afterhaving been exposed to actinic radiation to form cross-linked imagedareas of increased resistivity and non-imaged areas unexposed to theactinic radiation which remain the less resistive or background areas.The photosensitive material, such as a dry film resist, typically isformed of polymers which become cross-linked to form the imaged areas ofgreater electrical resistivity that may be an order of magnitude moredielectric than the background or unexposed areas. These imaged areasare the only areas of increased resistivity that hold a high voltagecharge when charged by a D.C. charge corona, if the conductive backingis electrically grounded. The non-imaged or background areas with thelesser electrical resistivity very rapidly release or leak the chargethrough the grounded conductive backing. The charged toner particlessuspended in the nonpolar insulating solvent are oppositely charged tothese latent imaged areas so that the charged toner particles areattracted to them. This then permits the transfer of these charged tonerparticles from the electrostatically imageable surface across the liquidgap to the receiving surface as previously described.

Once the toner image is formed by the toner particles in the imaged areaon a receiving surface, the particles are dried on the receiving surfaceby heating. This is illustrated diagrammatically in FIG. 2a as fusingfor the conductive receiving surface. The heat can be provided either bythe use of an oven or directed warm air through an air slot so that theheat is supplied for a finite period of time sufficient to reach thetemperature at which the binder or polymer forming the toner particleswill solvate in the liquid which is entrained within the transferredimage. The fusing, for example, with a copper circuit board can occurfor about 15 to about 20 seconds at a temperature greater than about100° C. and up to about 180° C.

Thereafter, in the instance of copper circuit boards, the non-imagedareas are etched to produce the desired conductive wiring pattern in theunetched conductive receiver surface which is overcoated with the tonerparticles. The etching step utilizes a solution that cannot remove theconductor material from the areas of the conductive receiving surfaceprotected by the toner particles, but does attack and remove theconductor material from the areas unprotected by the toner particles.The particular type of etchant employed depends, in part, on theconductor material being etched and the type of resist being used, sothat both acid and very mild alkaline etching solutions are possible foruse. For example, when the receiving surface is copper, an etchantcomprising acidic cupric chloride is preferably used.

The final step in the electrostatic transfer process using a conductivereceiving surface to form the copy is the stripping step, as seen inFIG. 2a. During this step the toner particles are appropriately removedor stripped from the imaged areas, such as by rinsing with methylenechloride, acetone, an alkaline aqueous solution or other suitablesolution.

In order to exemplify the results achieved, the following examples areprovided without any intent to limit the scope of the instant inventionto the discussion therein. The examples are intended to illustrate themanner in which a permanent master with a permanent latent image on theelectrostatically imageable surface can be obtained and how the gapspacing and voltage levels can be varied to achieve successfulelectrostatic image transfer. The examples also illustrate, whether aphotoconductor or a permanent master is used as the electrostaticallyimageable surface, how successful electrostatic image transfer can beachieved.

EXAMPLE 1

A liquid toner was prepared for use by preparing the following rawmaterials in the amounts shown in a high speed disperser:

    ______________________________________                                        Raw Material Amount (grams)                                                                             Description                                         ______________________________________                                        ISOPAR H     1248.6       solvent-carrier                                     UNIREZ 7059  439.2        alcohol insoluble                                   (UNION CAMP)              maleic modified                                                               rosin ester                                         Allied AC    307.8        linear polyethylene                                 Polyethylene 6A                                                               BAKELITE     1584.0       ethylene-                                           DPD 6169                  ethylacrylate                                       (UNION CARBIDE)           copolymer 20% shock-                                                          cooled suspension in                                                          ISOPAR H                                            phthalocyanine                                                                             229.2        coloring agent -                                    green                     pigment                                             Alkali Blue G                                                                              158.4        coloring agent -                                                              pigment                                             ______________________________________                                    

These components were mixed at a speed of 8000 rpm for 10 minutes whilemaintaining the temperature of the mixture between 160° and 220° F.

606 Grams of an amphipathic graft copolymer system was prepared bymixing 104.3 grams of lauryl methacrylate and 44.7 grams of methylmethacrylate, both available from Rohm and Haas, and 3.0 grams of azobisisobutonitrile, available from DuPont as Vazo 64.

Next 108.2 grams of an amphipathic copolymer stabilizer was preparedaccording to the procedure described hereafter. In a 1 liter reactionflask equipped with a stirrer, a thermometer and a reflux condensor isplaced 400 grams of petroleum ether (b.p. 90°-120° C.) and the same isthe heated at atmospheric pressure to a moderate rate of reflux. Asolution is made of 194 grams lauryl methacrylate, 6.0 grams of glycidylmethacrylate and 3.0 grams of benzoyl peroxide paste (60 percent by wt.in dioctyl phthalate) and placed in a 250 ml. dropping funnel attachedto the reflux condensor. The monomer mixture is allowed to drip into therefluxing solvent at such a rate that it requires 3 hours for the totalamount to be added. After refluxing 40 minutes at atmospheric pressurebeyond the final addition of monomer, 0.5 grams of lauryl dimethyl amineis added and the refluxing is continued at atmospheric pressure foranother hour. Then 0.1 gram hydroquinone and 3.0 grams methacrylic acidare added and refluxing continued under a nitrogen blanket until about52 percent esterification of the glycidyl groups is effected (about 16hours). The resulting product is slightly viscous straw-colored liquid.

345.8 Grams of ISOPAR H from Exxon Corporation was added to the 108.2grams of the amphipathic copolymer stabilizer and the aforementionedquantities of lauryl methacrylate, methyl methacrylate and azobisisobutnitrile to form the 606 grams of amphipathic graft copolymersystem. Polymerization was effected by heating this solution to about158° F. under a nitrogen atmosphere for about 4 to about 20 hours.

606 Grams of additional ISOPAR H was added to the above solution andmixing was continued for 10 minutes at 8000 RPM while the temperaturewas maintained between about 160° F. and 180° F.

Finally, 3578 grams of ISOPAR H was added, the mixer speed reduced to1000-2000 RPM, and mixing continued for 30 minutes. During this laststep, the temperature of the mixture was maintained between 120° F. and140° F.

Next a liquid toner concentrate was prepared by combining the followingin a static attritor-type mill:

    ______________________________________                                        Material    Amount (grams)                                                                             Description                                          ______________________________________                                        Predispersion Mix                                                                         1022.7       liquid toner                                                                  predispersion                                        Carnauba wax                                                                               58.3        wax                                                  polymer dispersion                                                                         83.3        amphipathic polymer                                                           dispersion as                                                                 prepared in Ex. XI                                                            of Kosel (U.S. Pat.                                                           No. 3,900,412)                                       Neocryl S-1004                                                                             62.4        amphipathic polymer                                                           dispersion                                                                    available from                                                                Polyvinyl Chemical                                                            Industries, Div. of                                                           Beatrice                                                                      730 Main St.                                                                  Wilmington, MA 01887                                 ISOPAR H     694.5       solvent-carrier                                      ______________________________________                                    

These components were milled for three hours at 300 RPM and atemperature of about 75° F. to create a toner concentrate. The tonerconcentrate was further diluted to about 1 to about 2 percent solids tocreate the working solution for use in electrostatic imaging.

A cadmium sulfide photoconductor overcoated with a MYLAR polyester filmlayer (typical of the NP process type) was corona charged and then lightexposed to a circuit trace pattern from about 0.75 to about 2.70microjoules/square centimeter in a Canon Model 1824 copier to create acharged latent image. The charged latent image was developed by applyingthe liquid toner to the overcoated cadmium sulfide photoconductor thatis the electrostatically imageable surface. The electrostaticallyimageable surface of this photoconductor is mounted over an inneraluminum substrate drum. The drum was removed from the copier. A highvoltage power source had its ground lead connected to the interior ofthe drum and its positive lead connected to the copper surface of theconductive receiving surface. Cellophane spacer strips or gap spacerswere used between the drum and the conductive surface. The conductivesurface was coated with a liquid that included the nonpolar insulatingsolvent that was squeegeed onto the conductive surface. Theelectrostatically imageable surface of the drum was coated during thedevelopment step. 1000 Volts D.C. current was applied and the gap wasset at 10 mils.

The cadmium sulfide drum was manually rolled across the spacer strips tocreate points of transfer of the latent image from the electrostaticallyimageable surface to the conductive receiving surface of copper. Theimage transfer was successful with the image possessing excellentresolution and good density.

EXAMPLE 2

A 4"×5" electrically conductive substrate of copper mounted on a glassepoxy support substrate known as FR 4, was selected as the conductivesubstrate for use in making the electrostatically imageable surface ofthe permanent master. The copper substrate was checked for need ofcleaning or degreasing. If necessary, the substrate can be cleaned withmethyl chloride, methylene chloride or trichloroethylene to promote goodadhesion of the photoresist to the cleaned surface during the subsequentlamination step. In this particular instance cleaning was not necessary.DuPont Riston 215 dry film photoresist was laminated to the substrate asthe photosensitive material. The lamination was accomplished with theuse of a Western Magnum Model XRL-360 laminator made by Dynachem ofTustin, Ca. The lamination was carried out at a roll temperature ofabout 220° F. and a speed of about six feet per minute. A protective toplayer of approximately 0.001 inch thick polyethylene terephthalate,hereafter PET, film was retained over the dry film photoresist of thecopper/Riston 215 laminate.

The laminate was exposed to actinic radiation through a negativephototool using the Optic Beam 5050 exposure unit manufactured byOptical Radiation Corporation. The exposure was accomplished after thelaminate cooled to room temperature following the laminating process.The exposure level was approximately 250 millijoules for about 60seconds. The phototool was a Microcopy Test Target T-10 resolution testchart, with groups of bars varying from 1.0 cycles or line pairs permillimeter to 18 cycles or line pairs per millimeter, sold byPhotographic Sciences, Inc.

The exposed electrostatically imageable surface was then allowed to coolto room temperature for about 30 minutes, thereby permittingcross-linking in the dry film to complete. The protective layer of PETfilm was peeled away. The copper substrate was grounded and theelectrostatically imageable surface was corona charged so that theimaged area received a positive charge. After a short delay of about asecond or more to allow background areas to discharge, the chargedpersistent image was then electrophotographically developed with liquidtoner of Example 1. Excess toner particles were rinsed from thedeveloped permanent master with Isopar H solvent carrier withoutallowing the toner to dry. The developed persistent image on theelectrostatic master was then ready for transfer to a conductivereceiving surface.

The electrostatic master thus formed was laid flat on a generally flatworking surface. MYLAR polyester spacer strips were placed along a pairof parallel and opposing edges of the master outside of the developedimage area to a thickness of about 10 mils.

A flexible conductive receiving surface of 1/2 ounce copper foillaminated to a 1 mil thick KAPTON.sup.(R) polyimide insulating layer waswrapped around and secured by lap taping the edges to a 1 1/2 inchdiameter drum. The receiving surface was wet with a layer of Isopar Hsolvent carrier by immersing the cylinder. Alternatively, the receivingsurface could be coated by pouring the liquid thereover.

An electrical potential of about 1000 volts was established to create anelectric field across an approximately 10 mil gap. The conductivereceiving surface of copper foil was charged with positive polarity withrespect to the electrically conductive copper substrate of the masterfor use with the negatively charged toner particles.

The 1 1/2 inch diameter drum with the conductive receiving surfacesecured thereto was rolled over the spacer strips on the edges of themaster. As the roller passed over the master at each discrete point oftransfer the toner particles were transferred from the master to theconductive receiving surface. The transferred image displayed excellentresolution up to about 3.6 line pairs per millimeter. It appeared asthough 100% of the toner particles were transferred to the conductivereceiving surface.

The conductive receiving surface was then exposed to a fan for up toabout 30 seconds to dry the non-imaged areas that comprise thebackground areas. The non-imaged areas should be dried while the imagedareas remain wet so the polymers in the toner particles can solvate inthe solvent carrier and not run outside of the imaged areas. An airknife can also be used to effect the drying of the non-imaged areas.

The transferred image on the conductive receiving surface was then fusedby placing in an oven for about 30 seconds. The temperature of the ovenprior to opening was about 180° C. The fusing is accomplished through atemperature ramping that effectively occurs when the oven door is openedto place the conductive receiving surface inside because of theresultant temperature drop within the oven. The oven temperaturegradually increases to the approximate 180° C. temperature level afterthe oven door is closed again.

EXAMPLE 3

A 1 mil thick transparent sheet of polyvinylfluoride, sold by DuPontunder the tradename TEDLAR, was placed on a vacuum platen to which aD.C. potential was applied. This sheet was the receiving substrate.

An electrostatically imageable surface was made by laminating twoone-mil (0.001 inch) thick films of DuPont 210 R dry film photoresist toa 0.3 mil thick aluminized polyester film layer of about 10 milthickness with a standard commercially available laminator, such asDynachem's Western Magnum Model XRL-360. The suitable polyester film canbe that sold by DuPont under the tradename MYLAR. The electrostaticallyimageable surface was then exposed to about 50 millijoules per squarecentimeter of actinic radiation using a negative phototool on an OptiBeam Model 5050 exposure unit manufactured by Optical RadiationCorporation. The phototool was an electrical circuit pattern. Thisexposed laminate, which serves as a permanent master, was allowed tocool and then was placed on the carrier web of a direct image transferdevice.

In an automatic sequence, the permanent master was corona charged at+6500 volts to give the image area a positive charge, toned with anegative acting Olin Hunt premium toner, designated as 750/770 toner, bydrawing it through a toner bath with about 1.5% solids in Exxon's ISOPARH non-polar insulating solvent, and then placed over the receivingsubstrate on the vacuum platen.

About 800 volts D.C. was applied to the vacuum platen, which served as aconductive electrode, and the carrier web, with the toned permanentmaster, was brought close to, but not in contact with, the receivingsubstrate. This was accomplished by means of a transfer roller thattraversed the back side of the carrier web while the receiving surfacewas kept in register to the electrostatically imageable surface. Theapplied voltage created the electrostatic field necessary to effect theelectrostatic transfer across the fluid-filled gap.

This gap was defined by one thickness of a suitable tape, such as ScotchBrand 810 Magic Transparent tape, placed on two opposing edges of thereceiving surface of the transparent sheet of polyvinylfluoride on thevacuum platen to give a gap of approximately 2 mils. The gap was filledwith ISOPAR H non-polar insulating solvent.

Approximately 95% of the toner was transferred from the permanent masteracross the fluid-filled gap to the receiving surface. While the vacuumand 800 volt D.C. transfer potential were still applied to the vacuumplaten, the wet toned image was partially dried on the receiving surfaceusing a heat gun to prevent spreading of the image. A high resolutionimage was obtained on the receiving surface.

While the preferred method in which the principles of the presentinvention have been incorporated is shown and described above, it is tobe understood that the present invention is not to be limited to theparticular details or methods thus presented, but, in fact, widelydifferent means and methods may be employed in the practice of thebroader aspects of this invention.

For example, to effect transfer the electric field established betweenthe electrostatically imageable surface and the receiving surface can becharged with either positive or negative polarity, depending upon thecharge of the toner particles, to direct the charged toner particlesacross the liquid medium. Charged toner particles of negative polaritywill be attracted to a positively charged receiving surface or will berepelled by a negative back charging of the electrostatically imageablesurface. If charged toner of positive polarity are used, they will beattracted to a negatively charged receiving surface or repelled by apositive back charging of the electrostatically imageable surface. Thenonpolar insulating solvent can equally well be mineral spirits, as longas it possesses high resistivity and low viscosity.

The gap spacing can equally well employ a web-to-web arrangement thatwill hold the electrostatically imageable surface and the receivingsurface at the desired distance.

The electric field can be established in several ways. For example, witha conductive receiver surface, such as the copper laminate, or in thecase of a dielectric material, such as MYLAR polyester film, backed by aconductive surface, the electric field is created by direct charging.Where a dielectric receiving surface, such as MYLAR polyester film, isused front or back charging via conventional corona charging or rollercharging can be employed.

The electrostatically imageable surface can be a photoconductor, such asa cadmium sulfide surface with a MYLAR polyester film or a polystyreneor a polyethylene overcoating, a selenium photoconductor surface, orsuitable organic photoconductors such as carbazole and carbazolederivatives, polyvinyl carbazole and anthracene. Where theelectrostatically imageable surface uses a persistent latent image as apermanent master, the surface can be zinc oxide, or organicphotoconductors developed with toner which is fused onto the master, ora dry film or liquid photoresist.

The type of photosensitive material applied to the conductive backing tomake the permanent master may vary as long as it is permanentlyimageable and possesses the correct resistivity characteristics. Forexample, where dry film resists are used, the films may be aqueous,semi-aqueous or solvent based. Photoconductive insulating films of zincoxide dispersed in a resin binder may also be used.

The process disclosed herein is equally well acceptable for use in theproduction of labels, high speed production of documents andphotochemical machining or milling. It may also be used to transfer animage to an absorbent substrate, such as paper. This may be effected byfirst heating the receiving surface, composed of a material such assilicone and then by contact transfer from the intermediate siliconereceiving surface to the absorbent substrate.

The scope of the appended claims is intended to encompass all obviouschanges in the details, materials and arrangements of parts which willoccur to one of skill in the art upon a reading of the disclosure.

Having thus described the invention, what is claimed is:
 1. A method offabricating a toned pattern on a nonabsorbent receiving surfaceelectrically isolated from ground, comprising the steps of:(a)establishing a charged electrostatic latent image area on anelectrostatically imageable surface; (b) developing the electrostaticlatent image area by applying to the electrostatically imageable surfacecharged toner particles suspended in a liquid comprised at leastpartially of a nonpolar insulating solvent to form a first liquid layerwith a first liquid surface, the charged toner particles being directedto the latent image area of the electrostatically imageable surface toform a developed latent image of a predetermined pile height; (c)applying to the nonabsorbent receiving surface a liquid comprised atleast partially of a nonpolar insulating solvent to form a second liquidlayer with a second liquid surface; (d) establishing an electric fieldbetween the electrostatically imageable surface and the nonabsorbentreceiving surface by connecting a D.C. voltage directly across aconductive electrode and the electrostatically imageable surface, theconductive electrode being connected to the nonabsorbent receivingsurface; (e) placing the nonabsorbent surface adjacent to theelectrostatically imageable surface so that a gap is maintainedtherebetween and the first liquid surface contacts the second liquidsurface to create a liquid transfer medium across the liquid-filled gap,the liquid-filled gap being of a depth greater than the pile height ofthe toner particles; (f) transferring the developed latent image fromthe electrostatically imageable surface through the liquid to thenonabsorbent receiving surface to form a transferred toner particleimage in an imaged area and define non-imaged areas where tonerparticles are absent; (g) maintaining the gap during transfer of thedeveloped latent image between the electrostatically imageable surfaceand the nonabsorbent receiving surface between at least about 1 mil andabout 20 mils.
 2. The method according to claim 1 further comprisingmaintaining the gap between the electrostatically imageable surface andthe nonabsorbent receiving surface at the point of transfer between atleast about 3 mils and about 10 mils.
 3. The method according to claim 1further comprising maintaining at a point of transfer a first planetaken through the electrostatically imageable surface parallel to asecond plane taken through the nonabsorbent receiving surface.
 4. Themethod according to claim 3 further comprising holding the nonabsorbentreceiving surface in register to the electrostatically imageablesurface.
 5. The method according to claim 4 further comprising holdingthe nonabsorbent receiving surface flat at the point of transfer.
 6. Themethod according to claim 5 wherein at least one of the receivingsurface or the electrostatically imageable surface is curved.
 7. Themethod according to claim 3 further comprising holding the nonabsorbentreceiving surface stationary at the point of transfer.
 8. The methodaccording to claim 7 further comprising holding the electrostaticallyimageable surface stationary at the point of transfer.
 9. The methodaccording to claim 7 further comprising moving the electrostaticallyimageable surface at the point of transfer in such a manner that thereis no relative motion between the electrostatically imageable surfaceand the nonabsorbent receiving surface at the point of transfer.
 10. Themethod according to claim 4 further comprising moving the nonabsorbentreceiving surface.
 11. The method according to claim 10 furthercomprising moving the electrostatically imageable surface at the pointof transfer in such a manner that there is no relative motion betweenthe electrostatically imageable surface and the nonabsorbent receivingsurface at the point of transfer.
 12. The method according to claim 10further comprising holding the electrostatically imageable surfacestationary at the point of transfer.
 13. The method according to claim 5further comprising using a vacuum to hold the nonabsorbent receivingsurface in place.
 14. The method according to claim 5 further comprisingusing a vacuum to hold the electrostatically imageable surface in place.15. The method according to claim 5 further comprising magneticallyholding the nonabsorbent receiving surface in place.
 16. The methodaccording to claim 1 further comprising forming a permanent latent imageon the electrostatically imageable surface.
 17. The method according toclaim 16 further comprising forming the permanent latent image in theelectrostatically imageable surface wherein the electrostaticallyimageable surface is selected from the group consisting of a dry filmphotoresist, a liquid photoresist, zinc oxide and organicphotoconductors.
 18. The method according to claim 1 further comprisingapplying between about 200 to about 3500 volts to the conductiveelectrode connected to the receiving surface to form the electric field.